U-Pb isotope geochronology of zircon: evaluation of the laser probe-inductively coupled plasma mass spectrometry technique

Hirata, Takafumi and Nesbitt, Robert W. (1995) U-Pb isotope geochronology of zircon: evaluation of the laser probe-inductively coupled plasma mass spectrometry technique. Geochimica et Cosmochimica Acta, 59 (12), 2491-2500. (doi:10.1016/0016-7037(95)00144-1).

Abstract

Using a laser ablation microprobe-inductively coupled plasma mass spectrometer (LP-ICPMS) we have determined 238U, 207Pb, 206Pb, and 204Pb abundances of several zircon populations whose ages have previously been measured by other techniques (principally ion microprobe). Ages of the samples range from 360–2800 Ma. A frequency quadrupled Nd-YAG UV laser (266 nm) which produces pit sizes of 10–15 ?m was used to ablate the zircon samples.

Prolonged measurements of the 206Pb/238U isotope ratio using ablation techniques show a serious fractionation effect because of nonconstant focusing of the laser. This ablation fractionation dramatically increases the Pb/U isotopic ratio because of the greater volatility of Pb. If the beam is constantly refocussed during ablation (active focusing), the effect of fractionation is minimised and the duration of the signal intensity increased.

207Pb/206Pb isotopic ratio measurements on zircons with ages greater than 2400 Ma previously measured by SHRIMP give an excellent agreement. Typical precision of the measurement for 207Pb/206Pb isotopic and Pb/U elemental ratios determined on single zircon grains is about 0.6–5% and 3–20%, respectively. In the case of Phanerozoic zircons (c. < 500 Ma), the LP-ICPMS allows the use of 206Pb/238U ratios and this provides more accurate data than the comparable 207Pb/206Pb system. The data presented here demonstrate clearly that the LP-ICPMS has a potential to become a significant and cost-effective tool for use in U-Pb zircon geochronology.

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